I can offer a few tips, having gone through a phase where everything I played sounded like Katatoina because I was so into them... I have a Digitech Rp-12 but these are some general ideas you wanna go for if you're after the Katatonia sound...
- You need to tune your guitar to down one half step to Eb...
- For distortion, don't use alot of gain, because most of their riffs are downpicked steadily and will sound like crud if you use too much overdrive... tweak your distortion so that you can hear each note as you play it instead of a blur... go for 'clean distortion.'
- On your amp, don't use alot of treble... use quite a bit of midrange and use less bass than midrange. If you were playing something like Nevermore or Testament, then you'd want more treble and bass in your sound...
- Alot of their clean guiar is played very very dry (no effects), high-midranged, on the bridge pickup, as opposed to Paradise Lost and other guys who usually use the middle-bridge or neck-middle combinations. (Opeth is the only other band which comes to my mind that does that bridge-clean thing as well...)
- For the 'Strained' sound, any chorus pedal will do, but don't turn the level all the way up... make the signal half distortion, half chorus... also, use a triangle waveform, not a logarithmic or 'detuned' setting, that's pretty much the secret.
And if your really going for the sound, on the amp, roll off a bit of treble, use a little bass, and boost the mids.
They also use other pedals like tremolo (the 'surf' volume chopping sound, not the picking technique) and delay...
There are tons of sounds that appear on their albums, but I can throw one more trick at you guys...
The freaky distortion that appears @ 1:46 in "12" is made by maxing out the gain on your signal and palm-muting the strings.. weird!!!
I think it's cool that Anders is a metal guitarist yet he's also into effects and ambience...
Yea, he's been known to be a huge Paradise Lost advocate, in fact, he lifted the bridge riff from PL's song "Weeping Words" and used it as the intro riff to "Inside The Fall" from the 'Sounds Of Decay' E.P.
I hope this helps you guys!!